Symmetric serrated edge light guide film having elliptical base segments

ABSTRACT

The present invention provides a planar light guide film for a backlight unit having at least one point light source, the light guide film comprising a light input surface for receiving light from the point light source, a light redirecting surface for redirecting light received from the light input surface and a light output surface for outputting at least the light redirected from the light redirecting surface. The light input surface further comprises a composite lens structure having a circular tip segment with a first contact angle, and a first and second elliptical base segments with a second contact angle, the second contact angle being greater than the first contact angle and the second contact angle being equal to each other and 
     wherein the circular tip segment satisfies the following equation: 
         y   1   =a   1 +√{square root over (( r   1   2   −x   2 ))}
 
     and the elliptical base segments satisfies the following equations: 
         y   4   =d   4   +b   4 ×√{square root over ((1−(( x+c   4 )/ a   4 ) 2 )}
 
         y   5   =d   5   +b   5 ×√{square root over ((1−(( x−c   5 )/ a   5 ) 2 )}

FIELD OF THE INVENTION

The present invention relates to a light guide film of a light emittingdiode (LED) backlight unit, and, more particularly, to a light guidefilm of an LED backlight unit, which has a plurality of grooves carvedinto an incident plane of the light guide film to increase an incidenceangle of which light can be transmitted through the light guide film.

BACKGROUND OF THE INVENTION

Typically, a liquid crystal display (LCD) for handheld and notebookdevices generally employs at least one lateral light emitting diode(LED) as a light source of a backlight unit. Such a lateral LED isgenerally provided to the backlight unit as shown in FIG. 1 of Yang U.S.Pat. No. 7,350,958.

Referring to FIG. 1, the backlight unit 10 comprises a planar lightguide film 20 disposed on a substrate 12, and a plurality of lateralLEDs 30 (only one lateral LED is shown in FIG. 1) disposed in an arrayon a lateral side of the light guide film 20. Light L entering the lightguide film 20 from the LED 30 is reflected upwardly by a minutereflection pattern 22 and a reflection sheet (not shown) positioned onthe bottom of the light guide film 20, and exits from the light guidefilm 20, providing back light to an LCD panel 40 above the light guidefilm 20. Such a backlight unit 20 suffers from a problem as shown inFIG. 2 when light is incident on the light guide film 20 from the LED30.

As shown in FIG. 2, light L emitted from each LED 30 is refracted towardthe light guide film 20 by a predetermined angle θ due to difference inrefractive index between media according to Snell's Law when the light Lenters the light guide film 20. In other words, even though the light Lis emitted at a beam angle of α1 from the LED 30, it is incident on thelight guide film 20 at an incidence angle of α2 less than α1. In FIG. 3,such an incidence profile of light L is shown. Therefore, there is aproblem of increasing the length (l) of a combined region where beams oflight L entered the light guide film 20 from the respective LEDs 30 arecombined. In addition, light spots H also called “hot spots” and darkspots D are alternately formed in the region corresponding to the length(l) on the incident plane of the light guide film 20. Each of the lightspots H is formed at a location facing the LED 30, and each of the darkspots D is formed between the light spots H.

Since the alternately formed light and dark spots are not desirable forthe light guide film, they should be minimized and the length (l) shouldbe shortened as much as possible. For this purpose, it is necessary toincrease an angle of light entering the light guide film, that is, anincidence angle of light.

For this purpose, it is suggested to form protrusions on the inputsurface of the light guide film as shown in FIG. 4. Specifically, aplurality of fine prism-shaped structures 24 or arc-shaped structures(not shown) are formed on a light input surface of a light guide film20A and light L enters the light guide film at an incidence angle α3substantially equal to an orientation angle α1 of light emitted from afocal point F of a light source. Thus, if orientation angles al of lightbeams emitted from the focal point F of the light source are identical,the light L enters the light guide film at an incidence angle α3 widerthan the case of FIGS. 2 and 3. However, with this solution, there issome secondary light collimation where the light rays are refracted bythe wall of the adjacent prism or arc-shaped structure as shown in FIG.4. Secondary light collimation from the walls of the adjacent prismstructure turns the light ray back on-axis providing less diffusion ofthe light from the light source as shown in FIG. 4. Thus the continuousprism- or arc-shaped structures on the input surface have limited lightdiffusing capability.

Therefore an improved input edge design is needed to provide a moreuniform surface illumination of the light guide film without sacrificingthe efficiency of the backlight system.

SUMMARY OF THE INVENTION

The present invention provides a planar light guide film for a backlightunit having at least one point light source, the light guide filmcomprising: a light input surface for receiving light from the pointlight source; a light redirecting surface for redirecting light receivedfrom the light input surface; a light output surface for outputting atleast the light redirected from the light redirecting surface; whereinthe light input surface further comprises a composite lens structurehaving a circular tip segment with a first contact angle, and a firstand second elliptical base segments with a second contact angle, thesecond contact angle being greater than the first contact angle and thesecond contact angle being equal to each other; and

wherein the circular tip segment satisfies the following equation:

y ₁ =a ₁+√{square root over ((r ₁ ² −x ²))}

and the elliptical base segments satisfies the following equations:

y ₄ =d ₄ +b ₄×√{square root over ((1−((x+c ₄)/a ₄)²)}

y ₅ =d ₅ +b ₅×√{square root over ((1−((x−c ₅)/a ₅)²)}

In addition, the invention further provides a planar light guide filmfor a backlight unit having at least one point light source, the lightguide film comprising: a light input surface for receiving light fromthe point light source; a light redirecting surface for redirectinglight received from the light input surface; a light output surface foroutputting at least the light redirected from the light redirectingsurface; wherein the light input surface further comprises a compositelens structure having gaps there between, the lens structure having acircular tip segment with a first contact angle, and a first and secondelliptical base segments with a second contact angle, the second contactangle being greater than the first contact angle and the second contactangle being equal to each other; and

wherein the circular tip segment satisfies the following equation:

y ₁ =a ₁+√{square root over ((r ₁ ² −x ²))}

and the elliptical base segments satisfies the following equations:

y ₄ =d ₄ +b ₄×√{square root over ((1−((x+c ₄)/a ₄)²)}

y ₅ =d ₅ +b ₅×√{square root over ((1−((x−c ₅)/a ₅)²)}

Further, the invention provides a planar light guide film for abacklight unit having at least one point light source, the light guidefilm comprising: a light input surface for receiving light from thepoint light source; a light redirecting surface for redirecting lightreceived from the light input surface; a light output surface foroutputting at least the light redirected from the light redirectingsurface; wherein the light input surface further comprises a serratedlens structure that is provided only where the point light source isincident on the light input surface, the lens structure having acircular tip segment with a first contact angle, and a first and secondelliptical base segments with a second contact angle, the second contactangle being greater than the first contact angle and the second contactangle being equal to each other; and

wherein the circular tip segment satisfies the following equation:

y ₁ =a ₁+√{square root over ((r ₁ ² −x ²))}

and the elliptical base segments satisfies the following equations:

y ₄ =d ₄ +b ₄×√{square root over ((1−((x+c ₄)/a ₄)²)}

y ₅ =d ₅ +b ₅×√{square root over ((1−((x−c ₅)/a ₅)²)}

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic diagram illustrating a conventional backlightmodule;

FIG. 2 shows a schematic diagram illustrating the distribution ofbright/dark bands of a conventional light guide plate;

FIG. 3 shows a schematic diagram illustrating an embodiment ofconventional light-diffusing structures;

FIG. 4 shows a schematic diagram illustrating another embodiment ofconventional light-diffusing structures;

FIGS. 5 a and 5 b shows a schematic diagram illustrating a light guidefilm according to an embodiment of the invention;

FIG. 6 a-6 c show schematic diagrams illustrating the various segmentsof the composite lens feature according to an embodiment of theinvention;

FIGS. 7 a and 7 b show schematic diagrams illustrating the lightdiffusing capability of the composite lens feature with a gap betweeneach adjacent feature;

FIG. 8 shows another embodiment of this invention;

FIGS. 9 a and 9 b show the luminance intensity at various distances fromthe light input surface for a circular or arc shaped input feature;

FIGS. 10 a and 10 b show the luminance intensity at various distancesfrom the light input surface for a trapezoidal feature or feature withslanted sides; and

FIGS. 11 a and 11 b show the luminance intensity at various distancesfrom the light input surface according to an embodiment of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

A light guide film in accordance with the present invention comprises alight output surface, a light redirecting surface and at least one lightinput surface that joins the light output surface and the lightredirecting surface. The light input surface comprises a plurality ofconcave features consisting of a composite lens array. Each of thecomposite lenses is separated by a gap that is a flat surfaceperpendicular to the light output surface. The composite lenses and gapsare disposed along the light input surface, and extend from the outputsurface to the light redirecting surface. Each of the composite lenseshas a symmetric cross-section consisting of a tip portion comprising acircular tip segment of a first contact angle and a base portioncomprising two tilted elliptical base segments with a second contactangle, the second contact angle being greater than the first contactangle and where the contact angle for each of the two tilted ellipticalbase segments are equal.

According to the above embodiment, the geometrical profile of thecomposite lens allows for comparatively large light deflectingdistances; that is, the composite lens structure has betterlight-diffusing capability. Thus, the distance between the point lightsource and the active area of the display can be shortened, and the darkspots between the point light sources can be minimized, with thebrightness uniformity still being acceptable. The circular tip segmentuniformly distributes the light in front of the discrete light source,typically a light emitting diode (LED). The two tilted elliptical basesegments uniformly distribute the light between the LEDs. A smoothcurvature of the circular tip segment and tilted elliptical basesegments maximizes the uniformity of the light spatial distribution sothat the light output is uniform. Further, it is also necessary thateach two adjacent composite lens structures have a gap or flattherebetween so a greater degree of deflection on the propagation pathof the incident light can be achieved to thereby increase thelight-diffusing effect.

Referring to FIGS. 5 a and 5 b, a light guide film according to anembodiment of the invention is shown, wherein a planar light guide film12 is used to receive and guide the light from at least one point lightsource (such as LEDs 14 shown in FIG. 5 a). The side surface of thelight guide film 12 next to the LED 14 forms a light input surface 12 a.The top surface of the light guide film 12 that makes an angle with thelight input surface 12 a forms a light-emitting surface 12 b, and thebottom surface opposite the light-emitting surface 12 b forms alight-reflecting surface 12 c. The light-reflecting surface 12 c iscomprised of a plurality of light reflecting structures. The lightemitted from the LED 14 enters the light guide film 12 via the lightinput surface 12 a and propagates inside the light guide film 12. Then,it is guided toward the light-emitting surface 12 b by thelight-reflecting surface 12 c and finally exits the light guide film 12through the light-emitting surface 12 b.

Further, a plurality of concave composite lens structures 16 areserrated on the edge of the light input surface 12 a, with theirlongitudinal directions being parallel to each other and having a gap(G) between each adjacent composite lens structure 16. Referring now toFIGS. 6 a, 6 b and 6 c, the light input surface 12 a, facing the LED 14,of the composite lens structure 16 has a circular tip segment 16 a, andtwo tilted elliptical base segments 16 b and 16 c, respectively. Thecircular tip segment 16 a of the concave composite lens structure 16 isthe segment furthest from the light input surface 12 a. Although thecomposite lens features for the preferred embodiment of this inventionare disposed in a concave direction on the light input surface, thecomposite lens may also be in a convex direction on the light inputsurface.

The length T₁ is the distance between the intersections of theextensions of the tangent at the top of the elliptical base segments 16b and 16 c, and the tangent of the circular tip segment 16 a, where thetangent of the circular tip segment 16 a is parallel to the light inputsurface 12 a. The length T₂ is the total width of the circular tipsegment 16 a taken where the circular tip segment 16 a intersects thetwo elliptical base segments 16 b and 16 c. Note, T₂ is parallel to T₁.The contact angle A₁ is the contact angle of the circular tip segment 16a. Contact angle A₁ is preferably greater than 0.1 degrees and less thanor equal to 85 degrees. Referring now to FIG. 6 b, the gap G is thedistance between each adjacent composite lens. Preferably, the gap G isless than or equal to 0.9 times the pitch P. The pitch P of the linearcomposite lens array 16 is the distance along the light input edge whichincludes the gap G distance and the total width B of the composite lens.Preferably the pitch P is greater than or equal to 5 micrometers andless than or equal to 1 millimeter (mm) The total height H of thefeature is measured from the light input edge to the tangent of thecircular tip segment 16 a. The total height H of the composite lens isgreater than or equal to 3 micrometers and less than or equal to 1millimeter. The light input surface 12 a will have a surface finish of10 nanometers to 2 micrometers. The surface finish of the concavecomposite lens structures 16 can be the same or different than the gap Gportion between the features.

Advantageously, the shape of an XY section of the circular tip segment16 a satisfies the following expression (1):

y ₁ =a ₁+√{square root over ((r ₁ ² −x ²))}  (1)

where the circular tip segment 16 a has a first radius r₁. The firstradius r₁ is defined as the quotient of half the distance T₁ divided bythe tangent of half the contact angle A₁. The value a₁ is defined as thetotal height H minus the radius r₁ of the circular tip segment 16 a. Thevalue x is a value in the direction of the light input surface and ispreferably set within the range of −r₁×sin(A₁)≦x≦r₁×sin(A₁). The valuey₁ is a value in the light propagation direction. Referring now to FIG.6 c, the composite lens structure also comprises two tilted ellipticalsegments, namely a first elliptical base segment 16 b and a secondelliptical base segment 16 c. Each elliptical base segment comprises twocontact angles, a top contact angle and a bottom contact angle. Thefirst elliptical base segment 16 b has a top contact angle A₄₁ and abottom contact angle A₄₂. The second elliptical base segment 16 c has atop contact angle A₅₁ and a bottom contact angle A₅₂. The top contactangle A₄₁ is created by a tangent to the first elliptical base segment16 b at the point where the circular tip segment 16 a and the firstelliptical base segment 16 b intersect. The bottom contact angle A₄₂ iscreated by a tangent to the first elliptical base segment 16 b at thepoint where the first elliptical base segment 16 b intersects the lightinput surface 12 a. The top contact angle A₄₁ of the first ellipticalbase segment 16 b and the top contact angle A₅₁ of the second ellipticalbase segment 16 c are equal. The bottom contact angle A₄₂ of the firstelliptical base segment 16 b and the bottom contact angle A₅₂ of thesecond elliptical base segment 16 c are equal. The contact angles foreach of the two elliptical base segments 16 b and 16 c are larger thanthe contact angle A₁ of the circular tip segment 16 a.

Advantageously, the shape of an XY section of the elliptical basesegments 16 b and 16 c as shown in FIG. 6 c satisfy the followingexpressions (2 and 3) respectively:

$\begin{matrix}{\mspace{79mu} {y_{4} = {d_{4} + {b_{4} \times \sqrt{( {1 - ( {( {x + c_{4}} )/a_{4}} )^{2}} }}}}} & (2) \\{\mspace{79mu} {{y_{5} = {d_{5} + {b_{5} \times \sqrt{( {1 - ( {( {x - c_{5}} )/a_{5}} )^{2}} }}}}\mspace{20mu} {{Wherein}\text{:}}\mspace{20mu} {H_{3} = {H - {r_{1} \times \lbrack {1 - {\cos ( A_{1} )}} \rbrack}}}\mspace{20mu} {B = {P - G}}{a_{4} = {{ab}_{4} \times \sqrt{\frac{( {B - T_{2}} ) \times \lbrack {{4H_{3}} + {( {{\tan ( A_{41} )} + {\tan ( A_{42} )}} ) \times ( {B - T_{2}} )}} \rbrack}{( {{\tan ( A_{41} )} + {\tan ( A_{42} )}} )}}}}{b_{4} = {2 \times {ab}_{4} \times \sqrt{\frac{H_{3} \times \lbrack {{H_{3} \times ( {{\tan ( A_{41} )} + {\tan ( A_{42} )}} )} + {{\tan ( A_{41} )} \times {\tan ( A_{42} )} \times ( {B - T_{2}} )}} \rbrack}{( {{\tan ( A_{41} )} + {\tan ( A_{42} )}} )}}}}\mspace{20mu} {{where}\text{:}}{{ab}_{4} = \frac{\begin{matrix}{( {{\tan ( A_{41} )} + {\tan ( A_{42} )}} ) \times ( {{2H_{3}} + {{\tan ( A_{41} )} \times ( {B - T_{2}} )}} ) \times} \\( {{2H_{3}} + {{\tan ( A_{42} )} \times ( {B - T_{2}} )}} )\end{matrix}}{\begin{matrix}{4 \times \lbrack {{4H_{3}} + {( {{\tan ( A_{41} )} + {\tan ( A_{42} )}} ) \times ( {B - T_{2}} )}} \rbrack \times} \\\lbrack {{H_{3} \times ( {{\tan ( A_{41} )} + {\tan ( A_{42} )}} )} + {{\tan ( A_{41} )} \times {\tan ( A_{42} )} \times ( {B - T_{2}} )}} \rbrack\end{matrix}}}{c_{4} = \frac{\begin{matrix}{{{\tan ( A_{41} )} \times \lfloor {{2H_{3} \times B} + {{\tan ( A_{42} )} \times ( {B^{2} - T_{2}^{2}} )}} \rfloor} +} \\{2H_{3} \times T_{2} \times {\tan ( A_{42} )}}\end{matrix}}{4 \times \{ {{{\tan ( A_{41} )} \times \lbrack {H_{3} + {{\tan ( A_{42} )} \times ( {B - T_{2}} )}} \rbrack} + {H_{3} \times {\tan ( A_{42} )}}} \}}}\mspace{20mu} {d_{4} = \frac{H_{3} \times \lbrack {{2H_{3}} + {{\tan ( A_{41} )} \times ( {B - T_{2}} )}} \rbrack}{{4H_{3}} + {( {{\tan ( A_{41} )} + {\tan ( A_{42} )}} ) \times ( {B - T_{2}} )}}}}} & (3)\end{matrix}$

Thus, the first elliptical base segment 16 b has a top contact angle A₄₁and a bottom contact angle A₄₂ and the second elliptical base segment 16c has a top contact angle A₅₁ and a bottom contact angle A₅₂.Referencing FIGS. 6 a and 6 c, and equation 2, the height H₃ of thefirst elliptical base segment 16 b is equal to the total height H of thecomposite lens feature 16 minus the radius r₁ of the circular tipsegment 16 a times the quantity 1 minus the cosine of contact angle A₁of the circular tip segment 16 a. The total width B of the compositelens feature 16 is equal to the pitch P of the composite lens arrayminus the gap G distance. Preferably gap G is greater than 0 and lessthan or equal to 0.9 times the pitch P. The pitch P is preferablygreater than or equal to 5 micrometers and less than or equal to 1millimeter. The height of the second elliptical base segment 16 c isequal to the height H₃ of the first elliptical base segment 16 b.

The parameter a₄ is equal to the parameter ab₄ times the square root ofthe quotient of the quantity of the total width B of the composite lensfeature 16 minus the total width T₂ of the circular tip segment 16 atimes the quantity 4 times the height H₃ of the elliptical base segment16 b plus the tangent of contact angle A₄₁ at the top of the firstelliptical base segment 16 b plus the tangent of contact angle A₄₂ atthe bottom of the first elliptical base segment 16 b the quantity of thetotal width B of the composite lens feature 16 minus the total width T₂of the circular tip segment 16 a divided by the quantity the tangent ofcontact angle A₄₁ at the top of the first elliptical base segment 16 bplus the tangent of contact angle A₄₂ at the bottom of the firstelliptical base segment 16 b.

The parameter b₄ is equal to 2 times the parameter ab₄ times the squareroot of the quotient of the quantity of the height H₃ of the firstelliptical base segment 16 b times the quantity the height H₃ of thefirst elliptical base segment 16 b times the tangent contact angle A₄₁of the top of the first elliptical base segment 16 b plus the tangentcontact angle A₄₂ at the bottom of elliptical base segment 16 b plus thetangent contact angle A₄₁ times the tangent contact angle A₄₂ times thequantity of the total width B of the composite lens feature 16 minus thetotal width T₂ of the circular tip segment 16 a divided by the quantityof tangent contact angle A₄₁ plus tangent contact angle A₄₂.

The parameter ab₄ is equal to the quotient of the quantity the tangentof contact angle A₄₁ at the top of the first elliptical base segment 16b plus the tangent of contact angle A₄₂ at the bottom of the firstelliptical base segment 16 b times the quantity twice the height H₃ ofthe first elliptical base segment 16 b plus the tangent of contact angleA₄₁ at the top of the first elliptical base segment 16 b times the totalwidth B of the composite lens feature 16 minus the total width T₂ of thecircular tip segment 16 a times the quantity twice the height H₃ of thefirst elliptical base segment 16 b plus the tangent of contact angle A₄₂at the bottom of the first elliptical base segment 16 b times the totalwidth B of the composite lens feature 16 minus the total width T₂ of thecircular tip segment 16 a divided by 4 times the following quantitiesthe quantity the height H₃ of the first elliptical base segment 16 btimes 4 plus the tangent of contact angle A₄₁ at the top of the firstelliptical base segment 16 b plus the tangent of contact angle A₄₂ atthe bottom of the first elliptical base segment 16 b times the totalwidth B of the composite lens feature 16 minus the total width T₂ of thecircular tip segment 16 a, plus the quantity the height H₃ of the firstelliptical base segment 16 b times the quantity the tangent of contactangle A₄₁ at the top of the first elliptical base segment 16 b plus thetangent of contact angle A₄₂ at the bottom of the first elliptical basesegment 16 b plus the tangent of contact angle A₄₁ at the top of thefirst elliptical base segment 16 b times the tangent of contact angleA₄₂ at the bottom of the first elliptical base segment 16 b times thequantity times the total width B of the composite lens feature 16 minusthe total width T₂ of the circular tip segment 16 a.

The parameter c₄ is equal to the quotient of the quantity the tangentcontact angle A₄₁ at the top of the first elliptical base segment 16 btimes the quantity twice the height H₃ of the first elliptical basesegment 16 b times the total width B of the composite lens feature 16plus the tangent contact angle A₄₂ times the quantity the total width Bof the composite lens feature 16 squared minus the total width T₂ of thecircular tip segment 16 a squared, that quantity plus twice the heightH₃ of the first elliptical base segment 16 b times the total width T₂ ofthe circular tip segment 16 a times the tangent of contact angle A₄₂ atthe bottom of the first elliptical base segment 16 b divided by thetangent contact angle A₄₁ at the top of the first elliptical basesegment 16 b times the quantity the height H₃ of the first ellipticalbase segment 16 b plus the tangent of contact angle A₄₂ at the bottom ofthe first elliptical base segment 16 b times the quantity of the totalwidth B of the composite lens feature 16 minus the total width T₂ of thecircular tip segment 16 a plus quantity the height H₃ of the firstelliptical base segment 16 b times the tangent of contact angle A₄₂ atthe bottom of the first elliptical base segment 16 b the quantities ofthe divisor times 4.

The parameter d₄ is equal to the quotient of the quantity twice theheight H₃ of the first elliptical base segment 16 b plus the of tangentcontact angle A₄₁ at the top of the first elliptical base segment 16 btimes the quantity the total width B of the composite lens feature 16minus the total width T₂ of the circular tip segment 16 a, the previousquantities times the height H₃ of the first elliptical base segment 16 bdivided by the height H₃ of the first elliptical base segment 16 b times4 plus the quantity plus the tangent of contact angle A₄₁ at the top ofthe first elliptical base segment 16 b plus the tangent of contact angleA₄₂ at the bottom of the first elliptical base segment 16 b times thequantity the total width B of the composite lens feature 16 squaredminus the total width T₂ of the circular tip segment 16 a.

The coordinate x is a value in the direction of the input edge or morespecifically in the direction of the total width of the composite lensfeature 16 and is preferably set within the range of −B/2≦x≦−T₂/2 forthe first elliptical base segment 16 b. The coordinate y₄ is a value inthe light propagation direction.

Referencing FIGS. 6 a and 6 c, and equation 3, the height H₃ of thesecond elliptical base segment 16 c is equal the height of the firstelliptical base segment 16 b. The total width B of the composite lensfeature 16 is equal to the pitch P of the composite lens array minus thegap G distance. Preferably gap G is greater than 0 and less than orequal to 0.9 times the pitch P.

$a_{5} = {{ab}_{5} \times \sqrt{\frac{( {B - T_{2}} ) \times \lbrack {{4H_{3}} + {( {{\tan ( A_{51} )} + {\tan ( A_{52} )}} ) \times ( {B - T_{2}} )}} \rbrack}{( {{\tan ( A_{51} )} + {\tan ( A_{52} )}} )}}}$$b_{5} = {2 \times {ab}_{5} \times \sqrt{\frac{H_{3} \times \lbrack {{H_{3} \times ( {{\tan ( A_{51} )} + {\tan ( A_{52} )}} )} + {{\tan ( A_{51} )} \times {\tan ( A_{52} )} \times ( {B - T_{2}} )}} \rbrack}{( {{\tan ( A_{51} )} + {\tan ( A_{52} )}} )}}}$  where: ${ab}_{5} = \frac{\begin{matrix}{( {{\tan ( A_{51} )} + {\tan ( A_{52} )}} ) \times ( {{2H_{3}} + {{\tan ( A_{51} )} \times ( {B - T_{2}} )}} ) \times} \\( {{2H_{3}} + {{\tan ( A_{52} )} \times ( {B - T_{2}} )}} )\end{matrix}}{\begin{matrix}{4 \times \lbrack {{4H_{3}} + {( {{\tan ( A_{51} )} + {\tan ( A_{52} )}} ) \times ( {B - T_{2}} )}} \rbrack \times} \\\lbrack {{H_{3} \times ( {{\tan ( A_{51} )} + {\tan ( A_{52} )}} )} + {{\tan ( A_{51} )} \times {\tan ( A_{52} )} \times ( {B - T_{2}} )}} \rbrack\end{matrix}}$ $c_{5} = \frac{\begin{matrix}{{{\tan ( A_{51} )} \times \lfloor {{2H_{3} \times B} + {{\tan ( A_{52} )} \times ( {B^{2} - T_{2}^{2}} )}} \rfloor} +} \\{2H_{3} \times T_{2} \times {\tan ( A_{52} )}}\end{matrix}}{4 \times \{ {{\tan ( A_{51} )} \times \lbrack {H_{3} + {{\tan ( A_{52} )} \times ( {B - T_{2}} )}} \rbrack \times H_{3} \times {\tan ( A_{52} )}} \}}$$\mspace{20mu} {d_{5} = \frac{H_{3} \times \lbrack {{2H_{3}} + {{\tan ( A_{51} )} \times ( {B - T_{2}} )}} \rbrack}{{4H_{3}} + {( {{\tan ( A_{51} )} + {\tan ( A_{52} )}} ) \times ( {B - T_{2}} )}}}$

The parameter a₅ is equal to the parameter ab₅ times the square root ofthe quotient of the quantity of the total width B of the composite lensfeature 16 minus the total width T₂ of the circular tip segment 16 atimes the quantity 4 times the height H₃ of the second elliptical basesegment 16 c plus the tangent of contact angle A₅₁ at the top of thesecond elliptical base segment 16 c plus the tangent of contact angleA₅₂ at the bottom of the second elliptical base segment 16 c thequantity of the total width B of the composite lens feature 16 minus thetotal width T₂ of the circular tip segment 16 a divided by the quantitythe tangent of contact angle A₅₁ at the top of the second ellipticalbase segment 16 c plus the tangent of contact angle A₅₂ at the bottom ofthe second elliptical base segment 16 c.

The parameter b₅ is equal to 2 times the parameter ab₅ times the squareroot of the quotient of the quantity of the height H₃ of the secondelliptical base segment 16 c times the quantity the height H₃ of thesecond elliptical base segment 16 c times the tangent contact angle A₅₁of the top of the second elliptical base segment 16 c plus the tangentcontact angle A₅₂ at the bottom of elliptical base segment 16 c plus thetangent contact angle A₅₁ times the tangent contact angle A₄₂ times thequantity of the total width B of the composite lens feature 16 minus thetotal width T₂ of the circular tip segment 16 a divided by the quantityof tangent contact angle A₅₁ plus tangent contact angle A₅₂.

The parameter ab₅ is equal to the quotient of the quantity the tangentof contact angle A₅₁ at the top of the second elliptical base segment 16c plus the tangent of contact angle A₅₂ at the bottom of the secondelliptical base segment 16 c times the quantity twice the height H₃ ofthe second elliptical base segment 16 c plus the tangent of contactangle A₅₁ at the top of the second elliptical base segment 16 c timesthe total width B of the composite lens feature 16 minus the total widthT₂ of the circular tip segment 16 a times the quantity twice the heightH₃ of the second elliptical base segment 16 c plus the tangent ofcontact angle A₅₂ at the bottom of the second elliptical base segment 16c times the total width B of the composite lens feature 16 minus thetotal width T₂ of the circular tip segment 16 a divided by 4 times thefollowing quantities the quantity the height H₃ of the second ellipticalbase segment 16 c times 4 plus the tangent of contact angle A₅₁ at thetop of the second elliptical base segment 16 c plus the tangent ofcontact angle A₅₂ at the bottom of the second elliptical base segment 16c times the total width B of the composite lens feature 16 minus thetotal width T₂ of the circular tip segment 16 a, plus the quantity theheight H₃ of the second elliptical base segment 16 c times the quantitythe tangent of contact angle A₅₁ at the top of the second ellipticalbase segment 16 c plus the tangent of contact angle A₅₂ at the bottom ofthe second elliptical base segment 16 c plus the tangent of contactangle A₅₁ at the top of the second elliptical base segment 16 c timesthe tangent of contact angle A₅₂ at the bottom of the second ellipticalbase segment 16 c times the quantity times the total width B of thecomposite lens feature 16 minus the total width T₂ of the circular tipsegment 16 a.

The parameter c₅ is equal to the quotient of the quantity the tangentcontact angle A₅₁ at the top of the second elliptical base segment 16 ctimes the quantity twice the height H₃ of the second elliptical basesegment 16 c times the total width B of the composite lens feature 16plus the tangent contact angle A₅₂ times the quantity the total width Bof the composite lens feature 16 squared minus the total width T₂ of thecircular tip segment 16 a squared, that quantity plus twice the heightH₃ of the second elliptical base segment 16 c times the total width T₂of the circular tip segment 16 a times the tangent of contact angle A₅₂at the bottom of the second elliptical base segment 16 c divided by thetangent contact angle A₅₁ at the top of the second elliptical basesegment 16 c times the quantity the height H₃ of the second ellipticalbase segment 16 c plus the tangent of contact angle A₅₂ at the bottom ofthe second elliptical base segment 16 c times the quantity of the totalwidth B of the composite lens feature 16 minus the total width T₂ of thecircular tip segment 16 a plus quantity the height H₃ of the secondelliptical base segment 16 c times the tangent of contact angle A₅₂ atthe bottom of the second elliptical base segment 16 c the quantities ofthe divisor times 4.

The parameter d₅ is equal to the quotient of the quantity twice theheight H₃ of the second elliptical base segment 16 c plus the of tangentcontact angle A₅₁ at the top of the second elliptical base segment 16 ctimes the quantity the total width B of the composite lens feature 16minus the total width T₂ of the circular tip segment 16 a, the previousquantities times the height H₃ of the second elliptical base segment 16c divided by the height H₃ of the second elliptical base segment 16 ctimes 4 plus the quantity plus the tangent of contact angle A₅₁ at thetop of the second elliptical base segment 16 c plus the tangent ofcontact angle A₅₂ at the bottom of the second elliptical base segment 16c times the quantity the total width B of the composite lens feature 16squared minus the total width T₂ of the circular tip segment 16 a.

The coordinate x is a value in the direction of the input edge or morespecifically in the direction of the total width of the composite lensfeature 16 and is preferably set within the range of T₂/2≦x≦B/2 for thesecond elliptical base segment 16 c. The coordinate y₅ is a value in thelight propagation direction.

The contact angles for the composite lens feature can be described whereA₄₁=A₅₁, A₄₂=A₅₂ and A₁≦A₄₂, A₄₁. Preferably, A₁≦A₄₂, A₄₁≦85 degrees.

FIG. 7 a is a ray tracing for an array of a single composite lensfeature 16 of this invention illustrating what happens to the light rayswhen the individual composite lens features are disposed on the lightinput surface 12 a in a contiguous manner such that there is no gap Gbetween adjacent composite lenses. FIG. 7 b is a similar ray tracing,but where the individual composite lens feature is separated by a gap Gbetween adjacent features. The gap G is preferably less than or equal to0.9 P where P (as shown in FIG. 6 b) is the pitch of the composite lensfeature on the input surface 12 a. In FIG. 7 a, where the composite lensfeatures are adjacent each other along the input surface, some of thelight rays will experience a secondary light collimation as they arerefracted when they reach the side of the adjacent feature. Thissecondary light collimation detracts from the diffusion capability ofthe composite lens feature 16. In FIG. 7 b, the composite lens featuresare separated by a gap G. The gap allows the light ray to continue in adiffuse manner and thus widens the angle at which the light propagatesin the light guide film. There is minimal secondary light collimationwhen the gap between features is incorporated into the composite lensfeature design. In this way, the wider angle of light helps to mitigatethe hot spots along the input surface of the light guide film.

Referring now to FIG. 8, the light guide film 12 in FIG. 8 shows thecomposite lens features 16 not disposed along the entire input surface12 a. Instead, the composite lens features 16 are disposed along thelight input surface 12 a in the region where the LED 14 light isincident. The luminance uniformity of the system is minimally affectedas the unpatterned region on the light input surface has minimal lightrays in this region.

EXAMPLES

FIG. 9 a shows a portion of the light input surface 32 of a light guidefilm 30 with an arc- or circular-type structure 36. The graph in FIG. 9b illustrates the light intensity for the light guide film 30 atdistances 3.5 mm, 4.5 mm and 5.5 mm from the light input surface 32.FIG. 9 b shows that the localized light intensity decreases as thedistance increases from the light input surface, but there are stillsome hot spots evident at 5.5 mm The arc- or circular-type structuresolution provides some improvement for hot spots but is more effectiveat collimating light in line with the LED than widening the incidenceangle. This is evident in the graph in FIG. 9 b. In FIG. 9 b, the LEDsare located at each of the vertical dotted lines and the lightdistribution is still not leveled out at 5.5 mm into the light guidefilm. It is apparent from the graph in FIG. 9 b that the arc- orcircular-type solution has insufficient diffusion capability.

FIG. 10 a shows a portion of the light input surface 42 of a light guidefilm 40 with a composite lens structure that has flat slanted sides 46.This result would also be applicable to a trapezoidal shaped light inputstructure. The graph in FIG. 10 b illustrates the light intensity forthe light guide film 40 at distances 3.5 mm, 4.5 mm and 5.5 mm from thelight input surface 42. FIG. 10 b shows that the localized lightintensity actually inverts in the area immediately in front of the LEDs,resulting in a dark spot immediately in front of the LEDs. This overallloss of light intensity immediately in front of the LED is due to thefact that the straight slanted walls diffuse the light more readilythrough the sides than through the tip. It is also noted that the shapeof the light intensity profile across the light guide film does notchange significantly as the distance increases from the input surface42.

FIG. 11 a shows a portion of the light input surface 52 of a light guidefilm 50 with the composite lens feature 56 of this invention. Thecomposite lens feature utilizes a circular tip segment and two tiltedelliptical base segments. The top and bottom contact angles for each ofthe two tilted elliptical base segments are equal. The top and bottomcontact angles for each of the two tilted elliptical base segments aregreater than the contact angle of the circular tip segment. The circulartip segment uniformly distributes the light in the area immediately infront of the LED. The two tilted elliptical base segments uniformlydistribute the light between the LEDs. The smooth curvatures of thecircular tip segment and the two tilted elliptical base segmentsmaximize the uniformity of the light spatial distribution so the outputlight is uniform. The graph in FIG. 11 b illustrates that the compositelens 56 of the present invention generates uniform light output acrossthe light guide film at distances of 3.5 mm, 4.5 mm and 5.5 mm from theinput surface 52.

Hence, an improved light guide film is provided with symmetric lightredirecting features to improve light output uniformity withoutsacrificing light input efficiency. Namely, the improved light guidefilm 12 having composite lens structure 16 provides enhanced lightdiffusion in the plane parallel to the light extraction plane and lightreflection plane (top and bottom surfaces), allowing greater lightredistribution between discrete light sources (light traveling outsidethe critical angle of planar un-serrated input edge), so that the lightoutput uniformity is improved. Moreover, the light distribution in theplane perpendicular to the light extraction plane and light reflectionplane (top and bottom surfaces) is minimized, so that the condition ofthe total internal reflection is minimized for the inputted travelinglight.

1. A planar light guide film for a backlight unit having at least onepoint light source, the light guide film comprising: a light inputsurface for receiving light from the point light source; a lightredirecting surface for redirecting light received from the light inputsurface; a light output surface for outputting at least the lightredirected from the light redirecting surface; wherein the light inputsurface further comprises a composite lens structure having a circulartip segment with a first contact angle, and a first and secondelliptical base segments with a second contact angle, the second contactangle being greater than the first contact angle and the second contactangle being equal to each other; and wherein the circular tip segmentsatisfies the following equation:y ₁ =a ₁+√{square root over ((r ₁ ² −x ²))} and the elliptical basesegments satisfies the following equations:y ₄ =d ₄ +b ₄×√{square root over ((1−((x+c ₄)/a ₄)²)}y ₅ =d ₅ +b ₅×√{square root over ((1−((x−c ₅)/a ₅)²)}
 2. The planarlight guide film of claim 1 wherein the composite lens structure has apitch P greater than or equal to 5 micrometers and less than or equal to1 millimeter.
 3. The planar light guide film of claim 2 wherein thecomposite lens structure has a gap G less than or equal to 0.9 times thepitch P.
 4. The planar light guide film of claim 1 wherein the compositelens structure has a total height H greater than 3 micrometers and lessthan or equal to 1 millimeter.
 5. The planar light guide film of claim 1wherein the circular tip segment of the composite lens structure has acontact angle A₁ greater than 0.1 degrees and less than or equal to 85degrees.
 6. The planar light guide film of claim 5 wherein the compositelens structure further comprises contact angles A₄₁, A₅₁, A₄₂, and A₅₂wherein A₄₁=A₅₁, A₄₂=A₅₂ and A₁≧A₄₂, A₄₁.
 7. A planar light guide filmfor a backlight unit having at least one point light source, the lightguide film comprising: a light input surface for receiving light fromthe point light source; a light redirecting surface for redirectinglight received from the light input surface; a light output surface foroutputting at least the light redirected from the light redirectingsurface; wherein the light input surface further comprises a compositelens structure having gaps there between, the lens structure having acircular tip segment with a first contact angle, and a first and secondelliptical base segments with a second contact angle, the second contactangle being greater than the first contact angle and the second contactangle being equal to each other; and wherein the circular tip segmentsatisfies the following equation:y ₁ =a ₁+√{square root over ((r ₁ ² −x ²))} and the elliptical basesegments satisfies the following equations:y ₄ =d ₄ +b ₄×√{square root over ((1−((x+c ₄)/a ₄)²)}y ₅ =d ₅ +b ₅×√{square root over ((1−((x−c ₅)/a ₅)²)}
 8. The planarlight guide film of claim 7 wherein the circular tip segment of thecomposite lens structure has a contact angle A₁ greater than 0.1 degreesand less than or equal to 85 degrees.
 9. The planar light guide film ofclaim 8 wherein the composite lens structure further comprises contactangles A₄₁, A₅₁, A₄₂, and A₅₂ wherein A₄₁=A₅₁, A₄₂=A₅₂ and A₁≦A₄₂, A₄₁.10. A planar light guide film for a backlight unit having at least onepoint light source, the light guide film comprising: a light inputsurface for receiving light from the point light source; a lightredirecting surface for redirecting light received from the light inputsurface; a light output surface for outputting at least the lightredirected from the light redirecting surface; wherein the light inputsurface further comprises a serrated lens structure that is providedonly where the point light source is incident on the light inputsurface, the lens structure having a circular tip segment with a firstcontact angle, and a first and second elliptical base segments with asecond contact angle, the second contact angle being greater than thefirst contact angle and the second contact angle being equal to eachother; and wherein the circular tip segment satisfies the followingequation:y ₁ =a ₁+√{square root over ((r ₁ ² −x ²))} and the elliptical basesegments satisfies the following equations:y ₄ =d ₄ +b ₄×√{square root over ((1−((x+c ₄)/a ₄)²)}y ₅ =d ₅ +b ₅×√{square root over ((1−((x−c ₅)/a ₅)²)}